The present invention relates to a shift control apparatus for a vehicle, which protects the driving mechanism of the vehicle and inhibits excessive loads from acting on a torque converter by preventing the shifting to an opposite direction gear when the speed of the vehicle exceeds a threshold value.
In a vehicle that includes a torque converter transmission, such as a forklift that requires a high net working rate and that is repeatedly frequently moved backward and forward, for example, a reverse (R) mode gear might be engaged before the vehicle stops. Thereby, a powerful engine braking force is exerted and the vehicle changes direction in a short time. The foregoing type of operation, however, gives rise to repeated loading and unloading forces on the drive transmission and can overload the torque converter. Therefore, solutions have been proposed for preventing reverse-direction gears from being engaged during times when the speed of the vehicle exceeds a predetermined value. FIG. 3 depicts one such solution.
In the apparatus of FIG. 3, a speed sensor 50 detects when the vehicle is stopped, i.e., when velocity=0. The signal from the speed sensor 50 is supplied to an electronic control unit ECU, which thereupon energizes two relays 51, 52. The contacts of the relays 51, 52 are supplied with battery current through the ignition switch 60 and contacts in the shift selector switch 53. Thus, as the shift selector switch 53 is shifted to a forward (F) position to move the vehicle forward, electric current flows through the F contacts of the switch 53 to a conductor 54. This current then flows through the energized (closed) contacts of the relay 51 to energize a first solenoid 55, which is assumed to permit or effect engagement of a forward gear. This same current also energizes the coil of the latching relay 56, which latches in the closed position as long as F is selected. Current thereby continues to be supplied to the solenoid 55 even after the vehicle is moving.
If the shift selector is later shifted into the reverse (R) position, the current path through the shift selector switch 53 to the conductor 54 is broken. Consequently, both the relay 56 and the solenoid 55 become de-energized, and the solenoid 55 can be re-energized only if the vehicle is brought to a stop, since only then are the relays 51 and 52 energized. Further, for the same reason, the reverse drive mode is not activated until the vehicle is brought to a stop. At that time, current is supplied through the energized contacts of the relay 52 to the solenoid 58 and coil of the latching relay 59. This keeps the solenoid 58 energized after the vehicle is in motion. In this manner, both F and R can be engaged when the vehicle is at rest; at other times, moving the shift selector from one position to the another position results in the de-energization of one of the drive solenoids 55 and 58.
From the foregoing, it will be understood that the relays 51 and 52 are controlled by a signal from the ECU in response to the vehicle speed. Accordingly, once the shift selector is moved from either the F or the R position, the latching relays 56, 59 become de-energized and the solenoids 55, 58 cannot be re-energized until the vehicle speed is again brought to zero (or some value below a threshold level). Thus, once the shift switch is shifted from one drive direction to another, the previous drive direction cannot be re-entered until the vehicle stops. This is disadvantageous because it limits the versatility of conditions under which the vehicle can be operated. For example, the vehicle operator cannot disengage the drive mechanism momentarily and then re-engage the drive mechanism in the same direction, since the FIG. 3 configuration does not allow that shifting sequence.
The present invention addresses the above-mentioned limitation and provides an improved shift control apparatus and method that prevents the shifting into a reverse-direction gear when the vehicle velocity exceeds a threshold value, but nevertheless allows the disengagement and re-engagement of the drive direction in the same direction even when the vehicle velocity exceeds the threshold value.
In preferred embodiments of the invention, the foregoing operation is attained by supplying current to the drive mechanism control solenoids from the shift selector through switches that are controlled to remain closed as long as either (1) the vehicle velocity remains below the threshold value or (2) if velocity is above the threshold value, the previously selected directional mode is re-selected. If the velocity exceeds the threshold value, the switch controlling the opposite direction solenoid is opened to prevent engagement of the drive mechanism in the opposite direction.